The present invention relates to a knocking sensor for detecting the knocking generated in an internal combustion engine.
It has been known generally that the engine output and the fuel consumption characteristics become optimum under light knocking condition at relatively low speed engine rotation range while a strong continuous knocking affects seriously the durability of the engine. Accordingly, it has been the usual practice to take into account the ignition timing, which is closely related to the knocking, and a device in which the ignition timing is advanced or retarded by detecting the knocking condition of the engine so as to maintain a light knocking condition and to improve the fuel consumption and the output characteristics had been proposed.
In such a type of device, a knocking sensor for detecting the knocking in the engine is indispensable and various knocking sensors are known heretofore. Among such various known knocking sensors, there is one to detect the knocking condition by the knocking sound irradiated from the engine at the occurrence of the knocking condition.
This knocking sensor usually uses a condenser microphone. An example of such a sensor is shown in FIG. 1, which comprises a vibrating film 2 secured the peripheral portion by a microphone body 1 and a rear plate 3 arranged in proximity at back of the vibrating film 2. The rear plate 3 is mounted onto the microphone body 1 via an insulating material 4. In this knocking sensor, the vibrating film vibrates by a produced sound of an engine and electrostatic capacity between the vibrating film 2 and the rear plate 3 varies. This varying signal is derived as an output to an ignition timing controlling device not shown in the drawing and the knocking is judged.
In such a known knocking sensor, since a very small variation of the electrostatic capacity is to be sensed, the thickness of the vibrating film 2 and the space between the film 2 and the rear plate 3 are made some tens of microns respectively. Due to such a very delicate construction, the output characteristics vary greatly by the variation of environmental condition such as the temperature, moisture, etc. Furthermore by the very delicate construction of the thin vibrating film 2 and the narrow gap between the film 2 and the rear plate 3, a fine working is required and inconvenience is unavoidable in the increase of cost of the products.
Further disadvantage of the conventional knocking sensor lies in that the manner of mounting of the knocking sound sensor on the body structure of the vehicle. It was usual practice to mount the knocking sound sensor directly on such a body structure, therefore the conventional knocking sound sensor is generally located at some distant position from the engine body in an engine room. The knocking sound sensor is influenced by the temperature in the engine room which varies over a wide range depending on the outer temperature and the driving condition and the detecting characteristics varies accordingly. Furthermore, sound other than the knocking sound, for instance, noise coming from the regulating valve system and the auxiliary equipments, and outer noise may be picked up and thus the detecting characteristic varies and the detecting sensitivity may become unstable and lowered.